Rubbery copolymers containing a majority of isobutylene units are well known for their low gas permeability, unique damping properties, and low surface energy; these properties make such copolymers particularly desired in applications such as tire innerliners. In order to have a better compatibility or co-curability with other elastomer components in the end applications, an unsaturated comonomer, and/or a comonomer containing reactive functionality is introduced into the isobutylene rubbery polymer. Previously known comonomers include isoprene and styrene. The copolymer may be partially brominated to give better compatibility.
The tire industry has always been interested in enhancing the barrier properties of tires. One way to improve the barrier properties is to mix them with layered clays to form an elastomeric nanocomposite. The layered clay can adapt to five different states in the base polymer.
The first state is “particle dispersion” wherein the clay particle size is in the order of microns but uniformly dispersed in the base polymer. The terms aggregate and agglomerate have been used to describe this state.
The second state is an “intercalated nanocomposite” wherein polymer chains are inserted into the layered clay structure, this occurring in a crystallographic regular fashion, regardless of the polymer to clay ratio. Intercalated nanocomposites may typically contain several layers of polymer between organoclay plates. An increase in the gallery spacing of the nanoclay, swollen with rubber, from a pristine state of about 0.3 to 0.7 nm up to about 2.0 to 6.0 nm can be considered as creating an intercalated condition.
The third state is a “flocculated nanocomposite.” This is conceptually the same as intercalated nanocomposites; however, the individual clay layers are sometimes flocculated or aggregated due to hydroxylated edge to edge interactions of the clay layers.
The fourth state is an “intercalated-flocculated nanocomposite.” The clay plates in the nanocomposite can be separated; however, tactoids or agglomerates can form that have a thickness in the range of 100 to 500 nm.
The fifth state is an “exfoliated nanocomposite.” In an exfoliated nanocomposite, the individual clay layers are separated within a continuous polymer by an average distance that depends on the clay concentration or loading in the polymer.
However, in producing an elastomeric nanocomposite, the incompatibility between the hydrophobic polyolefin elastomer and the hydrophilic inorganic clays can make it very difficult to achieve a good clay dispersion or exfoliation within the elastomer. Much effort has been make to modify the clay or the elastomer to make the two elements more compatible. It has been previously shown that better interaction between the functionality on the polymer and clay surface would lead to a higher degree of clay dispersion and exfoliation. This, in turn, will yield elastomeric nanocomposites with enhanced barrier properties.